The present invention relates to cellular communication systems, and more particularly to the operation of user equipment when control and data information are supplied by different radio units in a cellular communication system.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. As the mobile terminal moves from one cell to another, the network hands over responsibility for serving the mobile terminal from the presently-serving cell to the “new” cell. In this way, the user of the mobile terminal experiences continuity of service without having to reestablish a connection to the network. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
Present-day cellular communication systems are typically based on a homogenous network, mainly consisting of large macro cells, each cell having one transmitter/radio unit that serves the entire cell. In future cellular systems, heterogeneous network architectures can be expected comprising a mix of large macro and small pico/femto cells. Furthermore, there will also be situations where a specific cell has several radio units. Such solutions make it possible to utilize advanced multiple-input-multiple-output (MIMO) technology and beam forming schemes and thereby improve the entire system spectral efficiency.
FIG. 2 shows one such example involving a serving cell 200 with a main radio unit (MRU) 201, four remote radio units (RRU-1, RRU-2, RRU-3, and RRU-4), and three terminals denoted A, B, and C, respectively. Each of the remote radio units RRU-1 . . . RRU-4 is connected to the MRU 201 by means of a respective link 203-1 . . . 203-4 (collectively referenced as “203”). In this case, the main radio unit 201 is transmitting control channels (CCH) and common reference symbols (CRS) that are used by all terminals (A-C) connected to the cell 200. The CRS:s are used for demodulation of the CCH as well as for mobility measurements. The CRS:s are also used by each terminal for fine tuning in time and frequency synchronization operations. However, dedicated reference symbols (DRS) are used for data reception on data channels (DCH). The DCH and DRS could furthermore be transmitted from different remote radio units (RRU-1 . . . RRU-4) that might be closer to the terminal. In FIG. 2, terminal C gets both the DCH and the CCH from the main radio unit 201. By contrast, terminals A and B each receive the CCH from the MRU 201 while data originates from one of the remote radio units (i.e., in this example RRU-3). The RRU:s could be distributed over the entire cell 200, thereby creating a network architecture that supports the use of advanced MIMO and beam forming schemes. Such a solution as in FIG. 2 with DRS is allowed and already introduced in the Long Term Evolution (LTE) standard denoted LTE Release 8/9.
The inventors of the subject matter described herein have recognized one or more problems presented by arrangements such as that which is depicted in FIG. 2. For example, if data and control signals originate from different radio units (e.g., CCH originating from the MRU 201 and DCH originating from one of the RRU:s), these signals can arrive with different timing at the terminal. Typically, in such an arrangement as is shown in FIG. 2, the data from any given one of the RRU:s is time compensated by a network controlling node (in LTE, denoted the eNode B) in order to compensate for the delay in the cable/link 203 between the MRU 201 and the given RRU. However the terminals are likely to move around, and depending on a given terminal's exact location (see, e.g., the different locations of terminals A and B), this strategy may not achieve exact time compensation; in fact, it is expected that a time difference of, approximately 0.5-1 microsecond between the CCH and DCH is likely. In LTE systems, which use Orthogonal Frequency Division Multiplexing (OFDM), as long as the time difference between radio paths of a given signal is smaller than the cyclic prefix (CP) (4.7 micro sec in LTE) the CP itself can mitigate the effects of time dispersion. However, in situation described above, the classical time dispersion case is not presented because it does not involve different paths of the same signal, but rather different data (CCH+CRS vs. DCH+DRS) being transmitted from the two different radio units.
As mentioned above, each terminal relies on the CRS to synchronize its own timing and frequency, and there will be approximately up to 1 microsec difference between each terminal's expected timing (i.e., based on the CRS) and the actual DCH timing. Hence, once a terminal performs a Fast Fourier Transform (FFT) on the received signal based on CRS information, a significant frequency rotation over the resource elements (i.e., a group of sub-carriers over a predefined period of time) in the frequency-domain is introduced in the data channel (relative to the control channel). The same holds for frequency error, but the rotation will be over a resource element in the time domain.
The accuracy in transmission frequency between the main and remote nodes should be within ±100-200 Hz, but assuming different sign on the frequency error between main node (MRU 201) and a remote node (RRU-x) there will be a significant frequency rotation in time between the CCH (which the terminal uses as frequency reference) and the DCH. The above mentioned problem will introduce noise in the channel estimation process and thereby result in degraded receiver performance.
Therefore, there is a need for methods and apparatuses that are able to detect and compensate for timing and/or frequency errors that result when a terminal receives information from two different radio units and derives its own timing/frequency synchronization from only one of them.